In current semiconductor processes, the electrical characteristics of an integrated circuit (IC), for example a resistance, a capacitance, or the gain of a transistor, are still unable to have ideal values as the circuit design absolutely. These errors on the electrical characteristics may cause the efficiency of the IC degraded or incorrect operation. In conventional power supplies, for example a low drop-out (LDO) regulator or a DC-to-DC converter, a trimmer is used to trim the circuit so as to reduce the error of the output voltage which will require extra trim pads and thereby greater chip area and higher cost. To reduce the number of pads, U.S. Pat. No. 6,703,885 to Fan et al. proposed a trimmer method and device. However, this method and device still require at least one trim pad.